High speed temporal and spatial modulators are at the heart of many optical fiber and free space communication systems. The 1.55 μm region of the spectrum is most suitable for both optical fiber as well as free space communication, because 1.55 μm represents the lowest dispersion and low loss region of optical fibers, as well as being in the eye safe region of the infrared spectrum. As such, there is an emerging application for modulators in free space optical communications in the range of 1.55 μm. During the last decade or two, there has been a significant push for the development of optical free space communications, due to the advancement of lasers and compact optical systems. Therefore, there is a demand for modulator technologies enabling high bandwidth for insertion into free space optical connections.
Technologies such as liquid crystal spatial light modulators and microelectromechanical systems (MEMS) deformable mirror aberration generators are limited by the intrinsic switching speeds of the material from which they are fabricated, and are only capable of modulation rates between 10 and 100 kHz. Multi-quantum well (MQW) light modulators, on the other hand, are better suited for use in high speed modulation systems.
In particular, reverse biased p-i-n structures based on III–V MQW structures are ideal for use as high speed modulators for the 1.55 μm and other near IR wavelengths. Modulation rates of small III–V MQW devices have been measured to be greater than 10 GHz resulting in bit transfer rates between 1 and 10 Mbps. Also, MQW devices can be designed to operate at specific wavelengths, due to the inherent quantum mechanical properties of a quantum well growth. However, conventional MQW p-i-n modulator devices have only achieved intensity ON/OFF contrast ratios of approximately 1.7:1. Realization of an optical communication link requires significantly higher contrast ratios than those previously achieved.
What is needed, therefore, are MQW devices grown to operate in the “eye-safe” region of the IR spectrum, and that are capable of achieving reasonable quantum efficiencies and high contrast ratios in order to close a communication link by resolving the logical on or off state. In a more general sense, there is a need for technologies that enable high bandwidth free space optical communications.